Method of manufacturing a molded door skin from a flat wood composite, door skin produced therefrom, and door manufactured therewith

ABSTRACT

A method of manufacturing a hollow core door is disclosed, as well as a corresponding hollow core door. The method includes the steps of providing a solid flat door skin, moisturizing the flat skin, applying a conditioning resin thereto, pre-heating the flat door skin, and thereafter pressing the flat door skin between a pair of heated platens in a press in order to reform the flat skin into a molded skin including a plurality of panels defined therein. The press continuously closes in order to reform the flat skin into the molded skin, with the rate of press closure being a function of the determined hardness of the flat skin to be reformed. The resulting door skins have an improved bond strength, and are efficiently manufactured.

The disclosed invention is to a method for manufacturing a molded doorskin from a solid flat wood composite material, as well as the resultingskin and a hollow core door produced therefrom. More particularly, thedisclosed invention is to a method for manufacturing a molded door skinin which a flat or planar solid blank of wood composite material ispre-heated, moisturized, and heated in a press to a temperaturesufficient to soften the blank and during which the press platen(s) arepressure actuated to close, with the closing pressure thereafterincreased continuously to a preset limit in order to deform the blankinto a molded configuration suitable for a door skin and ultimately forbeing assembled into a hollow core door.

Hollow core doors are used in both interior and exterior applications.Many hollow core doors are made from door skins formed from woodcomposite materials. These wood composite materials may include particleboard, flake board, hard board, and medium density fiberboard (“MDF”).The wood composites often utilize a resin binder, which frequently is athermal setting resin, in order to maintain the wood fibers forming thecomposite in solid form.

A hollow core door may be of the “flush” type, that is one that is flator planar on both major surfaces (i.e. both door skins are flat and donot include panels molded therein). Alternatively, a hollow core doormay be of the “molded” type, that is one having a series ofthree-dimensional panels or the like formed or molded into the skins asthey are being manufactured.

Standard molded door skins are formed from a relatively thick non-solidmat or bat of material, which is thereafter compressed in a press to arelatively thin, final thickness. The mat can be produced from eitherdry or wet fibers. If the mat has a very high water content, with theresult that water is squeezed out during the pressing operation. Thepress may be a multiple platen press, having a series of skin formingpockets. Because the mat is in a flexible state prior to the pressingoperation, then the resulting solid skin has sharply defined featuresacceptable to consumers because the wood fibers can flow in order toconform to the mold. Due in part to the high capital costs involved tobuild plants to make molded skins, manufacturers frequently require thatindividual orders be for a large number of skins in order to permitmaximum operating efficiencies. Smaller orders become cost prohibitive.

Flush door skins on the other hand may be made in a similar manner asmolded skins, except that the original mat or bat of wood fibers ispressed flat, and is not three-dimensionally molded to form panels orthe like. Alternatively, a continuous belt press may be used for flatskins. Thus, a flush door skin originates from a relatively thicknon-solid mat or bat of wood-like fibers, which is pressed into a planaror flat shape to define the flush solid skin. This means that standardmedium density fiberboard, or hardboard may be used.

Standard molded door skins are relatively expensive, because the capitalcost is quite high due to the need for dies, presses, and the like. Theflat or planar skins used for flush doors, on the other hand, arerelatively inexpensive, but do not provide the aesthetic features oftendesired by consumers.

Because of the cost differential between a wood composite “flush” or“flat” skin and a wood composite “molded” skin, attempts have been madeto transform flush skins into molded skins in an effort to moreinexpensively produce molded door products. Such prior efforts have notresulted in commercially acceptable door skins, principally because theappearance of the surface has typically not been satisfactory. Priorefforts to transform flush skins into molded skins have generallyresulted in the final molded skin having a cracked, marred, or otherwiseaesthetically displeasing configuration and/or appearance.

It is apparent from the above that there exists a need in the art for amethod of manufacturing a molded door skin from a flat wood compositeblank which permits a standard flush or flat skin blank to be used asthe base material, and which results in a molded door skin havingfeatures and surface characteristics acceptable to consumers. Yet afurther need in the art is a molded door manufactured from flat woodcomposite door skins, which has suitable resistance to moisture, so thatthe door may be used for exterior applications.

It is a purpose of this invention to fulfill the above-described needsin the art. In commonly owned U.K. Application No. 9707318.3), there isdisclosed a method of making a hollow core door by reforming flush orflat door skins into molded skins via a press, wherein pressure appliedto platens of the press is increased in a series of steps. It has beenfound that the method of the prior application can be improved upon, asset forth below, to enable more efficient flat skin shaping, andimproved strength to the end product door.

The disclosed invention meets these and other needs in the art. It is apurpose of this invention to fulfill the above-described needs in theart.

A primary object of the disclosed invention is a method of manufacturinga molded door skin from a flush wood composite blank through applyingcontinuously increasing pressure to a moistened and softened flat blank,so that the resulting skin has molded features and surfacecharacteristics acceptable to consumers.

A further object of this invention is a molded door skin that isimpervious to moisture, with the result that the door will not distortand therefore will be suitable for exterior applications.

A method of manufacturing a door skin, according to one aspect of theinvention, comprises the steps of providing a solid wood composite flatblank (i.e. one that has already been compressed from its thick, loose,moisture-containing state). The blank is pre-heated and moisturized,coated with a sealer, and placed between platens of a heated press. Theplaten(s) are heated to a temperature sufficient to soften the resin inthe blank and thus to soften the blank, sufficient pressure is appliedto close the platens, and the pressure is continuously increased forthereby causing the blank to be deformed into a molded shape determinedby the configuration of the platens. The rate of closure of the press isdetermined by, and is a function of a number of characteristicsincluding, the hardness, density, density profile, depth of molding, andpercentage binder or resin content of the flush skin blank beingreformed. The molded and now reformed blank is then removed from betweenthe press platens.

A door skin, according to another aspect of the invention, comprises amolded medium density fiberboard three-dimensional blank. The blank hasa first portion with a first preselected thickness. The blank has asecond portion with a second preselected thickness. The second thicknessis less than the first thickness.

A door skin, according to a further aspect of the invention, comprises,after reforming in the press, a molded medium density fiber board threedimensional blank having a density of from about 550 to about 1,200kg/m³, which density is substantially uniform throughout the thicknessof the skin (i.e. plus/minus about 75 kg per cubic meter throughout theskin's thickness, preferably within about plus/minus 25 kg per cubicmeter).

A door, according to a still further aspect of the invention, comprisesa peripheral frame having oppositely disposed sides. At least a firstmolded skin is provided. The skin has first and second sides. The firstor exterior side of the skin has a moisture impervious barrier appliedwholly there about. The door skin second or interior side is secured toone of the frame sides. A second door skin is secured to the oppositeside of the frame.

It is also an object of this invention to provide a door skin, thatafter reforming into a molded skin, is stronger than a standard flush orflat skin blank and a molded skin. In certain embodiments, the reformedmolded skin has a bond strength of at least about 2.0 N/mm², andpreferably a bond strength of at least about 2.5 N/mm². This can and isoften double the internal bond after processing.

These and other objects and advantages of the invention will be readilyapparent in view of the following description and drawings.

FIG. 1 is an elevational view of a hollow core door including a pair ofopposed reformed skins (i.e. molded from flat skins) made in accordancewith an embodiment of this invention.

FIG. 2 is a fragmentary cross sectional view taken along line 2-2 ofFIG. 1.

FIG. 3 is a fragmentary cross sectional view of a flat or flush doorskin blank being positioned in a molding press in accordance with thisinvention, this Figure showing the flat or flush skin while still in itsflat shape.

FIG. 4 is a schematic flow chart of a process used in manufacturinghollow core doors of FIGS. 1-2 in accordance with certain embodiments ofthis invention.

FIG. 5 is a schematic diagram illustrating a solid flat skin being fedtoward an IR pre-heat station, moisturizing station, sealing station,pre-press station, and press station in the FIG. 4 process.

FIG. 6 is a cross sectional view of a reformed skin according to theFIG. 1-5 embodiment of this invention.

FIG. 7 is a mold pressure versus time graph according to an embodimentof this invention as per FIGS. 4-5, illustrating how pressure iscontinuously applied to the press platen(s) during pressing (reforming)of the flat skin, with the platens then being maintained at a constantpressure, and after which a downward slope indicates the platen(s)pressure being released for thereby opening the press.

FIG. 8 is a graph illustrating how, in certain embodiments of thisinvention; each reformed skin has a substantially constant densitythrough the entire thickness of the reformed skin.

FIG. 9 is a graph according to an embodiment of this invention,illustrating the rate of press mold platen closure as a function of thehardness of the pressed solid flat door skin to be reformed.

FIG. 10 is a schematic flow chart of a process and apparatus used inmanufacturing doors of FIGS. 1-9 according to an embodiment of thisinvention.

In the accompanying drawings, like reference numerals indicate likeparts throughout the several views.

Hollow core door 1, as best shown in FIGS. 1 and 2, according toembodiments of this invention is efficiently made to aestheticallyresemble standard molded hollow core doors which are themselves made toresemble a traditional method of solid wool doors made from stiles railsand panels. In accordance with this invention, door skins 7, 9 of door 1are not molded directly from non-solid mats or bats to formthree-dimensional molded panels 3 as in standard molded doormanufacturing. Instead, flush (i.e. flat or planar) solid compositeskins 10, as best shown in FIG. 3, that have already been pressed intotheir compressed rectangular flat-skin shape are provided, pre-heated,moistened, sealed, and then reformed in a platen inclusive mold press inorder to result in reformed molded skins 7, 9, each of which has panels3. Reformed molded skins 7, 9 are used to manufacture hollow core door1.

By reforming flat pressed blanks 10 in such a manner, the prior doorskin molding procedure (e.g. with dies, presses and the like that takenon-solid bats and press them into molded skins) is avoided. Thus,molded doors 1 can be made more efficiently and cost effectively, andthe resulting door skins may have a strength of more than twice that ofstandard molded skins, and more than twice that of standard-flush orflat skin blanks. Standard molded skins from Masonite Corporation, forexample, typically have a bond strength of about 1.4 N/mm², whilereformed skins 7, 9 according to certain embodiments of this inventionpreferably have a bond strength of at least about 2.0 N/mm², and mostpreferably a bond strength of at least about 2.5 N/mm².

In accordance with certain embodiments of this invention, it has alsobeen found important to control the rate of closure of platen(s) 17, 19of press 21, as best shown in FIG. 3, as a function of the hardness,density, density profile, depth of molding, and percentage binder orresin content of the blanks 10 to be reformed, and to continuously applyincreasing pressure, in a non-stepped manner or progressively smoothapplication, to platen(s) 17, 19 in press 21 as blank 10 is beingreformed. It has been found that the continual increase in pressure,with the rate of closure being controlled as a function of blank 10material composition, results in a more efficiently reformed skin 7, 9with less surface cracking, and enables the wood fibers of flat skin 10to flow more easily during reforming to their new positions in skins 7,9.

Surprisingly, it has also been found that adding conditioning resins(e.g. melamine or urea formaldehyde thermal curing resins) to solidblank 10 prior to pressing, results in a stronger end product skin 7, 9and a more aesthetically pleasing reformed skin 7, 9. It has been foundthat the addition of these resins allows the stretched or brokeninternal bonds, created when deforming the boards actually repairs thesefibers and eventually reforms bonds stronger than were originallyevident. The quantity of these resins can be varied to suit the finalperformance of the product requirements in terms of moisture resistanceand internal bond strength.

Referring to FIGS. 1-2, hollow core door 1 appears to be of the standardmolded type, but in reality is not. Door 1 is made using pressed flushor flat blanks 10, that are reformed in accordance with certainembodiments of this invention to form molded skins 7 and 9 having panels3. Door 1, on each major surface thereof, includes a plurality of threedimensionally formed panels 3 and corresponding raised planar portions5. Door 1 includes a pair of opposing reformed skins 7 and 9 (thataesthetically resemble conventionally molded skins) which define hollowcore area 11 there between. In interior door applications (e.g. FIG. 1),skins 7 and 9 of door 1 represent the outer major surfaces of door 1,while in exterior door applications (e.g. FIG. 2), melamine impregnatedcrepe paper or phenolic resin crepe paper 13 may be disposed entirely orwholly about the exterior surface of each skin 7 and 9. Paper 13provides a moisture impervious barrier minimizing water absorption bydoor 1.

Referring still to FIGS. 1-2, reformed skins 7 and 9 of door 1 areadhesively secured to door frame 15, such as with polyvinyl acetate(“PVA”). Those skilled in the art will recognize that frame 15 extendsabout the periphery of rectangular reformed skins 7 and 9 and door 1,and typically includes two parallel wooden stiles extending alonglongitudinal edges of the door and two parallel wooden rails at thebottom and top of the door. Skins 7 and 9 are spaced apart from oneanother by frame 15 to form hollow core area 11 which can be filled withfoam or the like.

FIG. 3, which will be described in further detail below, illustratessolid blank skin 10 as it is placed between platens 17 and 19 of moldpress 21. Blank 10 was previously formed, as known in the art, into theillustrated flush or flat blank 10. Typically, blank 10 is formed bypressing a relatively thick, non-solid wood fiber mat or bat into asolid flat door skin 10 having no panels molded therein. At press 21,flat blank (or flat skin blanks) 10 is reformed into molded skins 7 and9, which each include panels 3 molded thereinto. At press 21, bottomplaten 19 may remain fixed in place as to position, and upper platen 17may move vertically with respect to platen 19 in order to open and closethe press, respectively. In such a manner, when platen 17 is moveddownward to apply pressure on blank 10, male protrusions 23 (each ofwhich corresponds to a panel 3 to be formed) mate with correspondingfemale recesses 25, with blank 10 there between, so as to reform flatblank 10 into molded skin 7, 9 having panels 3 defined therein. FIG. 6illustrates a reformed skin 7, 9, having a panel 3 defined therein,after leaving press 21. In sum, press 21 reforms flat blanks 10 so as tomold a plurality of panels 3 thereinto.

Still referring to FIG. 6, after reforming in press 21, each skin 7, 9has opposed surfaces 31 and 33 formed of surfaces of the reformed skin.Each skin has planar first portion 35 and planar second portion 37.Portion 37, together with angled offset portions 39 and 41, form a panel3 in the reformed skin. Offset portions 39 and 41 preferably have aconfiguration facilitating removal of the skin 7, 9 from the platens ofpress 21 upon conclusion of the reforming process. Portions 35, 37, 39,and 41 have different thickness in certain embodiments due to stretchingand wood fiber flowing processes reforming blank 10 in the press. Forexample, with a blank 10 having a nominal thickness of between 2.5 mmand 5 mm, more preferably between 3.0 mm to 3.5 mm, first portion 35 andportion 37 of the reformed skin may have a reduction of about 10% fromits original thickness of slightly less than 4 mm, while offset portions39 and 41 have a thickness of from about 2.5 to 3.5 mm (preferably about3.0 mm). In certain other embodiments, portions 35, 37, 39, and 41 mayall have substantially the same thickness.

The manufacturing process described below enables reformed skins 7, 9 tobe made from solid pressed flat blanks 10 with the resulting skins 7, 9being aesthetically pleasing, cost effective to make, substantially freeof surface cracks, and substantially unmarred.

Referring now to FIGS. 4-5 and 10, solid and already pressed flush/flatdoor blank 10 is provided. Flat blank 10 is preferably a wood composite,such as medium density fiber (MDF) board or hard board, bound togetherwith thermal setting resin(s). MDF frequently has urea formaldehyderesin as the binder, which resin can be molded at temperatures ofbetween 320° F. to about 425° F. MDF solid flat door skin blanks 10 areavailable in various thicknesses and weights, ranging from about 3 mm to7 mm. In certain preferred embodiments, blank 10 is in the upperthickness range, so as to provide sufficient wood fiber for providingsharp and well defined features and to prevent surface cracking attransition areas. However, any thickness from about 3 to 7 mm willsuffice.

Solid flat blank 10 is received at loading station 45. Blank 10 has adensity of at least about 550 kg/m³, preferably from about 750 to 850kg/m³, and a thickness of from about 3 to 7 mm. Blank 10 has an initialweight of from about 340-600 grams (gms). Blank 10 has an initialmoisture content of from about 7-9%, preferably about 8% by weight.Typically all of the resin (e.g. melamine or urea formaldehyde) binderin blank 10 is not cured, because over curing can cause brittleness.Hence, manufacturers cure the blank 10 sufficient to attain hardnessspecifications, leaving some resin uncured. From about 5-20% (sometimesfrom about 10-15%) of the resin in flat blank 10 is uncured orundercured. The 5-20% uncured resin in flush blank 10 will later becured in the disclosed reforming process, with the result that reformedskins 7 and 9 attain a significantly higher hardness than is currentlyavailable with other molded skins.

Optionally, blank 10 may be brushed clean at cleaning station 46 (seeFIG. 10) in order to remove dirt, dust, and other potential surfacecontaminants.

Referring to FIGS. 4-5, blank 10 is then forwarded to infrared (IR)pre-heating station 47, which preheats skin 10 using IR radiation or anyefficient means of raising board temperatures. The IR preheater 47preferably has a series of upper and lower banks of IR lamps, betweenwhich blank 10 is positioned. The outputs of the IR banks areindependently controllably, in order to account for blanks 10 ofdiffering thickness, composition, etc., so that the blanks 10 are notheated too high. Preheating to a temperature of about 80° to 100° C.begins the initial preparation of the blank 10 and enhances the abilityto accept added moisture (e.g. steam, spray, or direct roller coating).Pre-heating station 47 causes blank 10 to lose from about 3-15 grams ofweight, as blank 10 is preheated for about 25-125 seconds, preferablyfor about 30-90 seconds. Blank 10 leaves pre-heater 47 with a moisturecontent of about 5-7%. Station 47 preheats at least a surface of theblank to a temperature of at least about 80° C.

Pre-heated flat blank 10 is then forwarded to moisturizing direct rollcoating station 49. Rolls 50 and/or 51, as best shown in FIG. 5, atstation 49 rotationally contact the skin and apply a coating of moisture(e.g. water or the like) to at least one major surface (and possibly twoin some embodiments) of blank 10. The moisture content of the blank isthus caused to increase to about 9-15%, preferably about 10-12%. Blank10 is maintained at a temperature of from about 80° to 100° C. duringmoisturizing so as to be receptive to the added moisture. In certainembodiments, roll 50 is a non-moisturized press roll, while roll 51applies moisture to blank 10. Moisturizer (e.g. water) which may containa surfactant to aid moisture absorption may be applied to one or bothmajor surfaces of blank 10, in an amount of from about 50-290 grams persquare meter, most preferably from about 80-120 grams per square meter.According to an alternative embodiment of this invention, steam may bedirected at blank 10 at station 49 in order to add moisture to the flatsolid blank 10.

In certain embodiments, major surface(s) of blank 10 may be sanded priorto moisturizing, in order to facilitate efficient increase in moisturecontent of the blank via more efficient moisture absorption through thesurface. Sanding removes the material at the surface of blank 10, withthe surface typically having a cured resin content exceeding the curedresin content of the interior. Removal of the surface resin facilitatesmoisture transmission into blank 10.

Moisturized blank 10 is then forwarded to double roll coater 55. Rolls57 and 59 contact blank 10 and apply conditioning resin and possiblyalso pigmented sealer to flat blank 10. The applied resin and sealerincrease the moisture content of blank 10 to about 12-14% by weight. Theconditioning resin may include water, with an additive of from about5-20% by weight of melamine or urea resin. From about 20-200 grams/m² ofconditioning resin is applied to blank 10 at station 55. Thus, whileblank 10 already has some resin maintaining the wood fibers in solidform and also uncured resin, additional resin is added at station 55.The added resin improves the ability of blank 10 to be efficientlyreformed, while also providing increased hardness to the resultingmolded skin. Surprisingly, should the resin originally present in blank10 be melamine-based, either melamine or urea resins can be added atstation 55, or should the resin originally in blank 10 be urea-based,also either urea or melamine resins can be added at station 55. Thetemperature of blank 10 at station 55 is such that the resins do not yetbegin to react or cure. In certain embodiments, roll 59 applies theconditioning resin to blank 10, while roll 57 applies a color-pigmentedseal coating.

Pigmented sealer (for example, having titanium dioxide pigment toprovide a white or alternative color), applied at station 55 by e.g.roll 57, creates a uniform colored surface on the reformed skin.Preferably, the pigmented sealer is applied to what is to be theexterior surface of the skin. The pigmented sealer thus causes theresulting skin to be primed. Doors formed from prior molded skins needto be primed, thus adding cost. Approximately 4-10 gms/m² of pigmentedsealer can be applied to blank 10 at station 55 by top roll 57.

After the color-pigmented sealer and additional resin have been appliedat station 55, flat blank 10 is forwarded to pre-press station 61 foradditional heating. Blank 10 is maintained at station 61 for a timeperiod of from about 20-60 seconds (preferably about 30 seconds) at atemperature from about 110-130° C. (preferably about 120° C.). Pre-pressstation 61 has a confined volume, so that the moisture in blank 10 doesnot readily evaporate into the atmosphere. The moisture remains in theblank as its temperature is increased. Station 61 is closed, so thatmoisture in blank 10 cannot readily escape blank 10. Pre-press station61 may be formed from spaced, opposed oil or electric heated platens,between which blank 10 is positioned.

For exterior door applications, after blank 10 leaves pre-press station61, it is forwarded to barrier applying station 62, as best shown inFIG. 10. At station 62, a barrier, such as melamine impregnated crepepaper or phenolic resin crepe paper 13, is applied to the major surfaceof blank 10 that will be the exterior door skin surface (i.e. face awayfrom the door's interior). A suitable paper may be purchased from AkzoNobel under their name SWEDOTEC™ flexible primer films TGPN and TXP.Alternatively, a cross linking polymeric resin system, forming amoisture barrier, may be applied at station 62 as a two component liquidsprayed or otherwise applied to the surface of the blank 10. Themoisture impervious barrier (e.g. crepe paper or cross-linked resin)also increases the hardness of the resulting skin, and provides abrasionresistance that is beneficial during shipping and installation. Afterthe barrier 13 has been applied, the blank 10 is forwarded to press 21.

In interior door applications, after blank 10 has been pre-heated andoptionally further moisturized at pre-press station 61, flat blank 10 isforwarded to press 21, which has upper platen 17 and stationary lowerplaten 19.

Press 21 is heated, preferably by recirculating oil or electricresistance elements with platens 17 and 19 being heated to a temperaturesufficient to prepare the resin in the blank 10, and to thereby preparethe blank. Press 21, as noted, may be vented, preferably via small ventholes v, as best shown in FIG. 3. The bottom platen is vented in orderto release steam, volatiles, and similar gaseous products generatedduring the pressing operation. Surprisingly, it has been found thatventing the press 21 results in a stronger end product skin than doesintermittent venting of the press. The holes v are sufficiently small indiameter to preclude wood fibers from blocking them and/or marring theresulting underside surface.

At press 21, flat blank 10 is placed between platens 17 and 19, as bestshown in FIG. 3. Platen 19 remains still or fixed in place, and pressureis applied to upper platen 17 in order to force platen 17 downwardtoward platen 19. As platen 17 is urged toward platen 19, blank 10 isreformed in conformance with the shape defined by the interface of theplatens 17 and 19 and their corresponding elements 23 and 25 (e.g. inthe shape of a panel 3).

In alternative embodiments, both platens may be simultaneously movedtoward one another, or the bottom platen 19 may be moved upward towardplaten 17 which may be fixed.

As best shown in FIG. 7, the pressure applied to the platen(s) to closepress 21 (e.g. the pressure applied to platen 17) is uninterruptedlyincreased (see upward sloping portion 63) to a preset limit of pressureand or physical stops to control thickness. The applied pressure 63(which may be applied continuously and monotonically and/or in a linearmanner in some embodiments) causes platen(s) 17 and 19 to closerelatively slowly by as little as 0.25 mm per second with some material.Blank 10 is correspondingly relatively slowly reformed, until theconfiguration of FIG. 6 is achieved and press 21 closed. When the press21 is closed, the platens 17 and 19 are held at the pressure 65 of FIG.7 in the closed position, preferably for a period of from about 10-60seconds, most preferably from about 20 to 30 seconds. During the holdingstep, the wood fiber in the blank 10 continues to flow while the blank10 achieves its final configuration. In addition, the resin, both theoriginal uncured resin and resin added with the conditioner, begins toreact and cure. Curing the resins causes them to harden; thussolidifying the reformed blank 10 into skin 7. The plateau orsubstantially planar pressure portion 65 of FIG. 7 graph illustrates thesubstantially constant pressure applied to the platen(s) of the press 21during holding or cure time. Then, after this curing time has elapsed,press 21 is opened along 67, e.g. by lifting platen 17 upward, so thatreformed door skin 7, 9 can be removed therefrom. In FIG. 7, thedownward sloping pressure portion 67 illustrates the opening of press21. Preferably, pressure portion 67 in FIG. 7 slopes at a much greaterangle than does portion 63, indicating that the pressure release duringopening is much quicker than the pressure application during pressclosing.

During pressure portion 65, when peak pressure is being applied to theblank 10 via the platens, as much as 1200 pounds per square inch ofpressure may be applied, although this substantially constant pressureis preferred to be from about 600 to 900 pounds per square inch, andmost preferably about 750 pounds per square inch. Platens 17 and 19 areeach preferably a hard chrome plated steel die, preferably having aRockwell hardness of 60 to 70 or greater. The surfaces of the platenshave the hard chrome plating in order to resist accumulation of woodsugars, which otherwise might occur. Each platen is preferably fromabout 3-5 inches thick, preferably about 4 inches, with each platenbeing electrically heated, such as by a Kalrod, although oil circulationor steam circulation may be acceptable heat media in certainembodiments. Platens 17 and 19 are preferably mirror images of oneanother, with one being male and the other female. Preferably, eachplaten 17, 19 is maintained at a temperature of from about 320-425° F.,and most preferably from about 370-380° F., although this may vary byfiber and resin type, during the pressing process. The selectedtemperature, which is a function of the resins and the blank 10thickness, should be maintained throughout the pressing operation duringwhich flat blank 10 is reformed, in order to assure that the bindingresin in the blank melts/re-melts and remains flowable during thepressure application portion 63.

As best shown in FIG. 9, it has been found that for optimum reforming,the rate of press 21 closure should be controlled as a function of atleast one of the hardness density, density profile, depth of molding,and percentage binder or resin content characteristic of the blank 10being reformed. The harder the blank 10, the slower the press 21 closurerate. The closure rate of the press 21 is substantially constant incertain embodiments, and depending upon the hardness of the blank 10,the closure rate may vary from about 0.25 mm/second to 1.0 mm/second. Ithas been found that if press 21 closes too quickly, resin bonds withinthe blank 10 can break. Thus, the harder the solid blank 10, the slowerthe heated platens should close, in order to avoid substantial resinbond breaks during reforming.

When press 21 opens, and the reformed skin is removed, the result is thedoor skin 7, 9 illustrated in FIG. 6, each having a plurality of panels3 formed or molded therein as shown in FIG. 1. Those skilled in the artwill recognize that features other than panels 3 may be molded into theskin.

FIG. 8 illustrates that reformed skin 7, 9 has a substantially constantdensity throughout its thickness. This is a byproduct of the uniquemethod of manufacture described above. The density of skin 7, 9throughout substantially its entire thickness is preferably from about800 to 1,200 kg/m³, but higher than the density of original flush blank10 by around 10%.

Referring to FIG. 4, from press 21, the reformed molded skins 7, 9 maybe forwarded to optional reconditioning station 69, where the skins areremoistened to a moisture content of about 8% (if they were below thatafter leaving the press). Trimming may also be performed at station 69after remoisturizing. Remoistening at station 69 may be achieved bywater misting or the like, or by passing the reformed skin through awater bath. Priming is not necessary, if the pigmented sealer applied atstation 55 or pigmented pre press sealers are used. On all otherapplications priming is preferred. Once reconditioned, which isoptional, skins 7, 9 are forwarded to door forming station 71, at whicheach skin is adhesively secured to a door frame, preferably a woodenframe, in order to form a hollow core door 1. Should the door be anexterior door, then a further moisture impervious barrier may be appliedat station 73 to the exposed edges of the frame by edge banding orpainting. Those skilled in the art will recognize that the door 1 needonly have one molded skin 7 or 9, and that the opposite side may beflat.

The final door 1 is shown in FIGS. 1-2 as described above, with crepepaper 13 being used only in exterior door applications.

FIG. 10 illustrates an assembly line for carrying out the methoddescribed above. There are two presses 21, each having a pre-pressstation 61. This is because, in full operation, presses 21 operateslower than their corresponding pre-press stations 61. Furthermore, foreach loading station 45, moisturizing station 49, andsealing/conditioning station 55, etc., there may be a plurality ofpre-press stations 61, each of which then has two presses 21.Consequently, the assembly line of FIG. 10 is readily expandable;substantially reducing the initial capital costs required to produce inquantity door skins.

1. A method of manufacturing a hollow core door, the method comprisingthe steps of: providing a solid wood composite flat blank having adensity of at least about 550 kg/m³; moisturizing the flat blank toraise its moisture content; applying a liquid conditioning thermalcurable resin to at least one major surface of the flat blank;pre-heating at least a surface of the flat blank to a temperature of atleast about 80° C.; after said pre-heating step has been performed,positioning the flat blank into a press between first and secondplatens, at least one of the platens being heated; reforming the flatblank into a molded door skin including a plurality of panels definedtherein by closing the press; and affixing at least a first molded doorskin to a first side of a door frame and affixing a door skin to anopposite second side of the door frame. 2-17. (canceled)
 18. A hollowcore door comprising: a door frame; first and second door skins attachedto said door frame so as to define a hollow core area there between, atleast one of said skins being a molded door skin; said one molded doorskin having molded therein a plurality of panels; and wherein said onemolded door skin has a bond strength of at least about 2.0 N/mm². 19-22.(canceled)